Artificial Human vision - KSOS

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422 Kerala Journal of Ophthalmology Vol. XXI, No. 4 segmentation) or a scene understanding approach, attempting to extract information that can be used. • Transmitter is the link from the camera/ image processing components to the stimulator and electrode array,( which are usually located inside the body). Percutaneous or transcutaneous connections can be used for this connection 6 . A transcutaneous connection uses radiofrequency telemetry to send data and power to the embedded stimulator where as percutaneous devices uses direct cables. The transcutaneous devices have the advantages of reduced risk of infection. Even though earlier devices were made for Percutaneous connections the new generation ones are transcutaneous. This is being used in the current trials of new generation intelligent medical implant systems and by other researchers. • Electrodes are thin wires, which allow a small amount of precisely controlled electrical current to pass through it to stimulate the neurons. There are two main types of electrodes discussed in the literature; surface electrodes, which lie flat against the stimulation target and penetrating electrodes, which are inserted inside the stimulation target. Stimulator, located outside the body receives information from the transmitter and send through multiple electrodes. Subretinal Prosthesis are different from all other forms of artificial vision devices in terms of location of implantation, its energy needs, proximity to the inner retinal layers, existence in the subretinal atmosphere, heat generation etc. the details of this will be described later in this article. Types of artificial vision devices Different methods of implants are tried in different parts of the visual pathway starting from retina, optic nerve and optic radiation. (a) Cortical prosthesis; (photograph) In 1929, Forester noticed that cortical stimulation with electrodes caused the subject to see a spot of light in a position that depended on the site of stimulation 7 . Later in sixties and early seventies Brindley and Lewin worked on this and implanted an array of 80 platinum electrodes in a 52-year-old legally blind. Stimulation of these electrodes produced discernible phosphenes and also found that phosphenes moved with eye movements and that phosphene perception usually (but not always) stopped when stimulation ceased 8 . Dobelle worked on this and came out with similar reports of stimulus perception. In late seventies he used a 64-channel platinum electrode surface stimulation prosthesis and showed that it can allow blind patients to recognize 6-inch characters at 5 feet (approximately 20/1200 visual acuity) 7 . Patients in these initial experiments complained of an inability to appreciate distinct phosphenes, but rather reported seeing “halos” surrounding each of these phosphenes 9 . This approach has advantage of the potential to restore vision to the largest number of blind patients as it bypasses all diseased visual pathway neurons rostral to the primary visual cortex.But the disadvantages includes the difficulties encountered in controlling the number of phosphenes induced by each electrode, and interactions between phosphenes, need for high currents and large electrodes that can induce pain due to meningeal stimulation and occasional focal epileptic activity following electrical stimulation. There are two methods for cortical stimulation discussed. The one with surface stimulation electrodes and other with intra cortical electrodes. Intra cortical stimulation was introduced in the hope of remedying the shortcomings of surface cortical stimulation via a lower current and higher fidelity system. This employed smaller electrodes closer to the target neurons, therefore requiring less current and resulting in a more localized stimulation. Initial studies, during which the intra cortical prosthesis was implanted in humans for a trial period of 4 months, demonstrated the ability to produce phosphenes which exhibited colour. Current models of the intra cortical prosthesis which are being studied in animal models include the Illinois Intra cortical Visual Prosthesis project and the Utah Electrode Array 10 . In 2000 Dobelle reported a case report of a patient who has stayed implanted with his cortical prosthesis for 20 years 9 . This cortical implant system from the Dobelle institute was made commercially available (not approved by the FDA) and there was an article in the Wall Street Journal, which reported a 33-year-old
December 2009 Arshad Sivaraman et al. - Artificial Human Vision 423 female recipient who paid US$100,000 for the Dobelle system and was only able to use it for 15 min per day (as it was tiring and caused severe head ache).But the future of this device is in question as the major emphasis is on other types of artificial vision devices like epiretinal subretinal or optic neuronal 11 . b. Optic Nerve Prosthesis The optic nerve is an interesting and appealing site for the implementation of a visual prosthesis where the electrodes can be implanted on the temporal side of the optic nerve and the simulator placed on the orbital cavity. Advantages of this approach are that the entire visual field is represented in a small area and this region can be reached surgically. The disadvantages being that the dura mater has to be dissected with a possible risk of infection and possible interruption to the blood flow to optic nerve 18 . Also the optic nerve being a dense neural structure with approximately 1.2 million axons confined within a 2-mm diameter cylinder with the entire visual field represented, it can be difficult to achieve focal stimulation of neurons corresponding to the needed visual areas. Again it can be used only when intact retinal ganglion cells are present and therefore limited to the treatment of outer retinal (photoreceptor) degenerations only. Thus it can only be substitute to subretinal or epiretinal prosthesis. Reports of such a chronically implanted optic nerve electrode connected to an implanted neuro stimulator and antenna that resulted in phosphene perception in a volunteer with no residual vision due to retinitis pigmentosa was there 1 . It was encouraging as the blind volunteer was able to adequately interact with the environment while demonstrating pattern recognition Fig. 1. The schematic presentation of a visual prosthesis based on penetrating optic nerve electrical stimulation. (image courtesy Dr Quishi Ren ) and a learning effect for processing time and orientation discrimination. Further works are being carried out in this approach by various scientific groups. One such approach( fig. 1) in its early phase is being conducted by prof Quinshi ren in China where they have successfully implanted their prosthetic device in animal models and are planning for human experiments 12 . c. Intraocular Approaches (i ) Epiretinal Prosthesis Epiretinal implants specifically target surviving ganglion cells by positioning stimulating electrodes in close proximity to the inner surface of the retina (fig 2). Fig. 2. Artists view of an Epi retinal implant in position. (image Courtesy; Dr Hornig, IMI implants.) The electrodes will be implanted on the retinal surface after a pars plana vitrectomy with a retinal tack to hold it in position. Some surgeons fix the electrodes after an ILM peeling and others with out. The electrodes which are connected to the simulator can either be sewn outside the sclera with wire connections traversing through the pars plana wound (Prof Richard) or it can be secured in the capsular bag of the lens after lens removal, making it completely intra ocular (Prof Walter). As the electrodes are implanted near to nerve fiber layer the image processing work which is usually taken care of by inner retinal cells and its connections have to be addressed. Also the energy send to the nerve fibers should be ideally corresponding to natural out flow. This stays technically difficult as much of the details regarding image processing inside the retinal layers are unknown. To overcome this issue at least to a certain level, image processing is being done by the image processors.
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